The present invention relates to a method and apparatus for coating metal wire or fibrous materials with metal. The invention relates also to a scrim which may be woven from such a composite wire and the use of the scrim in the electrodes of electrochemical cells. More particularly, the present invention relates to a method and apparatus capable of producing continuous lengths of fine composite wire comprised either of metal wire coated with an extrudible, corrosion-resistant metal or a fibrous core material coated with an extrudible, corrosion-resistant metal such as lead, the weaving of that composite wire into a scrim, and the use of that scrim as an electrode grid in an electrochemical cell.
Conventional batteries include electrodes having metallic substrates on which a layer of active material is deposited. The battery may contain several pairs of positive and negative electrodes, stacked, rolled or suspended within a battery case and covered by the electrolyte contained within the battery case. Most conventional rechargeable batteries are of the lead acid type. The electrode grids of conventional lead acid batteries are coated with an active material, usually a lead oxide. The active material in the negative electrode contains expanders which allow the plate to become spongy when it is formed. A "forming" charge is applied to both positive and negative plates to convert the layer of active material on the positive plate to a porous oxide of lead and the layer of active material on the negative plate to sponge lead.
Conventional state of the art lead acid batteries are relatively heavy, causing the battery to have a low specific energy. The specific energy of some commercially available, state of the art lead acid batteries is on the order of about 14 watt hours per pound at the three hour discharge rate. The heavy weight of the battery is a direct consequence of the use of large amounts of lead in the electrodes, both in the grid and in the active material, and in the connectors and straps, or bus bars, of the battery.
Thick lead grids are required for several reasons. For instance, the active material usually takes the form of a paste which is cured onto the grid. Although the paste adheres well to itself, it does not adhere well to the electrode grid, particularly during repeated charge-discharge cycles. Because of this characteristic, the grid must be made more substantial so that it can act as a latticework to help support the active material.
Further, the electrode grid itself is relatively fragile, necessitating a construction which is heavier than needed for the grid to perform its electrical function. The grid used in many conventional lead-acid batteries is formed either by casting liquid lead into a mold of the desired configuration or by expanding sheet lead into a mechanically stiff grid. The grid is then assembled into the electrode assembly.
During the manufacture of the electrode, its handling, and its assembly into the battery, it is subjected to a number of mechanical stresses. Once assembled into the battery, the electrode will be subjected to a number of induced stresses. The primary source of manufacturing stress is the pasting operation, during which the paste of active material is troweled onto and into the grid. As it is troweled into and onto the grid, the paste, which is heavy and relatively stiff (i.e., not very plastic), tends to bend, stretch and tear the latticework. This deformation of the grid structure results in many points at which the lead in the grid is stressed, and it is at these stress points where corrosion will occur first and proceed at the fastest rate. Thus, expanded metal grids, which offer the advantage of being lighter than cast grids, are inherently susceptible to accelerated stress corrosion because each point at which the metal sheet was expanded represents a stress point.
The induced stresses are the result of factors such as volumetric growth and shrinkage of the electrode during battery charge and discharge cycles, sagging of the conductors due to the pull of gravity on the heavy mass of active material which they support, and, if the battery is used in an application such as in an automobile, vehicle shock, thermal cycling, and vibration. Mechanical failure of the electrode occurs when the mechanical and induced stresses to which the electrode is subjected exceed the tensile and/or shear strength of the materials comprising the electrode. To help prevent premature mechanical failure of the electrodes due to these stresses, it is necessary to manufacture them in thicknesses which enable them to withstand the stresses to which they are likely to be subjected. Because so much lead must be used to provide the thickness which enables the grid to withstand these stresses, conventional grids have cross-sectional dimensions that are much larger than is required for actually conducting electrical current. The result of the use of thick grids is a battery which is relatively heavy, and has a low specific energy and material utilization factor.
In addition to a thick grid, a thick coating of active material on the electrodes is necessary to increase battery capacity. The thick coating is necessary because, as a general rule, the thicker the layer of active material, the greater the capacity of the battery to store electrical energy.
As the volume (and weight) of active material is increased and the weight of the battery case, straps, posts and grid conductors remains relatively constant, the utilization of the active material is increased on a per unit weight basis. This increased utilization results in an increase in the specific energy of the battery. However, the active material utilization factor of many state of the art lead acid batteries is only approximately 50-55% of battery weight.
The thickness of the layer of active material requires that the grid latticework be strong enough to support this thicker layer of active material. Because of the relatively low tensile strength of pure lead, it is necessary to make the lead grids substantially thicker than would be necessary to enable them to serve their electrical function so that the grids will withstand the above-described mechanical and induced stresses.
There are limits to the thickness of the layer of active material. One limitation is imposed by the weight of the active material. Another limitation is a result of the electrical characteristics of the active material. The active material on the positive electrode is a semiconductor, that is, due to its own internal resistance, it is capable of conducting electricity only a relatively short distance through itself. Consequently, the thickness of the active material is limited to that distance through which the active material is capable of efficiently conducting current. This characteristic of the active material is one reason for the presence of the grid conductor in the positive electrode. The grid conductor serves the function of conducting the current generated in the active material out of the active material.
The thickness of the layer of active material is also limited by the relative inability of the active material to adhere to the grid during charge-discharge cycling and by its low tensile strength. As a result of these characteristics, the above-described mechanical stresses often cause the grid to prematurely shed the fragile active material. Additional battery weight results from the fact that precautions must be taken to prevent any active material from floating loose in the electrolyte in the cell and shorting out the battery. The electrodes may be provided with special glass compression pads to compress the active material against the grid, thereby preventing a short circuit in the cell, but also adding to the weight of the battery without improving battery capacity.
As a result of these factors, most of the batteries available to date represent a balance between durability, capacity and specific energy, with certain of these factors being optimized for certain applications. For instance, in applications in which the weight of the battery is the most important concern, the electrodes are manufactured with the thinnest layer of active material practical and grids are pared down as much as is made possible by the reduced thickness of the active material. A battery with a thin layer of active material and relatively light weight grids represents a trade-off of increased manufacturing costs, shorter battery life and lower capacity for lighter battery weight.
Another problem, also related to the weight, bulk and capacity of lead acid batteries is the fact that these factors make it difficult to construct a rechargeable battery in flashlight battery sizes, i.e., "D cells", "C cells" and so on down to "AAA cells" and smaller, special purpose batteries. Spiral wound lead acid cells are easily connected in series to form batteries, which are available in sizes ranging from "BC" to "D." These cells produce high currents and are constructed from lead grids which are die cut from a flat sheet of pure lead which is rolled inside the round battery case in a tight spiral. Each spiral grid has a relatively large surface area, and there is no need to connect several small grids in parallel as in a conventional battery, resulting in a savings in weight and the cost of manufacture, and these cracks are highly susceptible to attack by the acid, causing the rapid corrosion of the grid.
However, no cells in sizes below "D" are available commercially because the soft, thick (about 0.04 inches) grid used in these cells cannot be rolled into a spiral which is tight enough to be used in batteries of smaller diameter. Cracks and stress points form on the grids of batteries of this size because the radius of curvature of the grid exceeds that associated with the maximum strength of the lead. These cracks are highly susceptible to attack by the acid of the battery, resulting in the rapid corrosion of the grid.
These same problems are involved in the construction of conventional nine volt rechargeable batteries. At present, the only readily available, multi-purpose rechargeable nine volt battery is constructed of nickel and cadmium (the "nicad" battery). This battery contains six 1.25 volt cells and produces about 7.5 volts. It is unusable for some nine volt applications because of its low voltage. However, the more serious limitation of the nicad battery is its so-called "memory." A nicad battery which is repeatedly discharged at low currents will occasionally "forget" that it is capable of delivering high currents, a result of the chemical conditions within the cells. Although reversible, this characteristic results in the decrease of the life of the battery if the battery is left on float or standby charge for indefinite periods.
There have been many attempts to improve upon the basic scheme governing the construction of electrochemical cells. One approach attempts to make advantageous use of the fact that the outer surface (approximately the outer 5.times.10 inches) of the lead grid element provides all the electrical conductivity and surface area required for attachment of the active material. Hence, most of the lead in a conventional electrode grid does not participate in the electrochemical function, but merely provides the strength and stiffness for the grid to survive its environment and manufacturing stresses. This approach is characterized by the building of electrodes which provide only an outer layer of lead on the grid element by depositing a layer of lead on a substrate which possesses the desired properties of light weight and high mechanical and chemical durability.
This approach is exemplified by, for instance, U.S. Pat. No. 4,275,130. This patent discloses a battery in which the electrodes take the form of parallel stacked biplates composed of a thermoplastic material such as polypropylene with conductive fibers of carbon or metal embedded in it to serve as strengthening and conducting elements. Each biplate is provided with parallel stripes of lead in electrical contact with the conductive fibers to serve as a grid. The active material is held between thin, porous glass mats, and the stacked assembly is then axially compressed and assembled into the battery case.
This approach is also exemplified by U.S. Pat. No. 3,808,040, which discloses a method of manufacturing battery plate grids which involves the strengthening of a conductive lattice work to serve as a grid element by depositing strips of synthetic resin on the lattice work. This patent is hereby incorporated into the present disclosure by reference thereto.
U.S. Pat. No. 3,973,991, incorporated herein by reference thereto, discloses a light-weight electrode assembly comprised of at least three layers. The center layer, or grid, is formed of a thin, perforated sheet of a conductive metal such as lead, and the layers on either side of the grid are comprised of a paste of active material with short synthetic fibers or a mixture of synthetic and natural fibers suspended in the paste to help hold it together. The disclosure of this patent indicates that the center, conductive layer may be as much as 90% open area. However, a lead grid with this much open area might be much too fragile to manufacture at reasonable cost or to use in many applications, a fact which is implicitly recognized by the examples set forth in the disclosure of that patent which describe a thin layer of only approximately 56% open area.
U.S. Pat. No. 3,556,855 discloses a grid element comprised of an electrically conductive resin and metal coated glass fibers. The fibers are of short length, are dispersed in the resin, and have a layer of an electrically conductive metal deposited on them according to methods known in the art. Only silver, copper and nickel-coated glass fibers are disclosed by this patent, which is hereby incorporated herein by reference thereto.
U.S. Pat. No. 4,091,183 discloses a solid plate lead core grid with a special surface profile. This grid element is comprised of a solid core of lead sandwiched between two porous envelopes which contain the active material of the electrode and are provided with a special surface configuration. Electrodes of this type are assembled into a battery which is capable of resisting plate deformation during hard and deep discharges, but which represents no weight or capacity advantage.
This approach is also represented by U.S. Pat. No. 3,560,262, which discloses a non-woven nylon wafer with a thick coating of conductive metal electroplated onto it for use as a grid in the electrodes of alkaline batteries. The nickel hydroxide or cadmium hydroxide active material is deposited into the pores of the conductive metal.
Other attempts to improve on the prior art batteries have focused on the active material. For instance, U.S. Pat. No. 3,466,193 discloses a positive electrode comprised of a paste of lead dioxide containing 5-25% weight short lead fibers, spaced throughout the paste such that the fibers do not contact each other. The fibers are of relatively short length and are obtained by chopping lead wool. This paste is deposited on a frame made of a plastic resin to form the positive electrode, which is then assembled to a negative grid and inserted into the battery case, resulting in a battery which contains only about 29% less lead than many conventional batteries and which has a somewhat improved specific energy.
U.S. Pat. No. 4,110,241 discloses a method for making an active material reinforced with synthetic fibers. This reinforcement may increase the durability of the battery, but has little effect on its capacity or specific energy.
Similar problems of battery life, capacity and weight are encountered in other types of batteries, and attempts to improve these batteries, for the most part, represent adaptations of the above-described balancing formula to the specific type of battery. For instance, U.S. Pat. No. 3,770,507 discloses a resinous grid impregnated with lead dioxide and synthetic fibers for use in non-rechargeable primary batteries using fluoroboric acid as an electrolyte. U.S. Pat. No. 3,397,088 discloses an electrode that is enveloped in fibrous inorganic material such as potassium titanate paper and compressed so that the active material is forced into the pores of the fibrous sheet. Alternatively, the fibers may be dispersed throughout the active material. This improvement has particular application to high energy density and rechargeable batteries using cadmium and nickel or silver. U.S. Pat. No. 3,703,413 discloses a method of making inorganic fibers, such as zinc oxide fibers, which may be incorporated into the electrodes of zinc-containing batteries. None of these approaches have application to metal acid batteries, particularly lead acid batteries. And, like the approaches summarized above, all represent a trade-off between durability, capacity and specific energy.
Another approach which has been used to decrease the weight of the lead acid battery is to utilize a lead alloy in the electrode. Lead alloys such as antimony may be used to give strength to the battery's electrodes, but such batteries are still relatively heavy and the presence of the alloy may result in increased corrosion and gassing. For instance, although antimony adds strength to the electrode and increases the resistance of the grid to shedding of the active material under deep cycling, it is highly susceptible to corrosion and gassing. Further, the presence of antimony is undesirable because it promotes self-discharging. Calcium alloy is not as susceptible to corrosion, but the presence of calcium reduces the electrical conductivity of the grid by causing a calcium-based plaque to form around the positive grid conductors. This plaque formation is non-reversible and increases with age and the number of battery charge-discharge cycles, causing a gradual and permanent decrease in battery capacity.
Although all of these approaches have their merits, all represent a trade-off between weight, capacity and durability, and as far as is known, none have provided the light-weight, long-life battery necessary to allow the commercialization of such developments as the electrically-powered automobile on a large scale.
One approach which could provide significant weight reduction without impairment of the capacity and durability of the battery is the coating of a light-weight, high tensile strength fiber with sufficient lead such that the resulting composite wire would be suitable for use in the grid of the electrode. For instance, U.S. Pat. No. 3,808,040, summarized above, notes that the method for strengthening a conductive latticework disclosed by that patent may be used on a tissue of lead-coated glass fibers. However, to date, no wire with a coating of lead of sufficient thickness, purity and continuity has been available. As noted above, pure lead wire is not strong enough to use in lead acid batteries, and previous attempts at coating stronger materials with lead have been unsuccessful in making a product which may be used in such a battery. For instance, U.S. Pat. No. 275,859 discloses an apparatus for the extrusion of lead onto a core material, in particular, a telegraph cable. However, the apparatus disclosed in that patent is not capable of developing the high extrusion pressures necessary to extrude lead onto a core material of small enough diameter to be capable of being used for this purpose. The problem is complicated by the fact that there are few materials with the desired characteristics of high corrosion and high tensile strength, among others, which can survive chemical attack in a lead oxide-sulfuric acid battery.
Short fibers composed only of lead are disclosed in U.S. Pat. No. 3,466,193, also summarized above. U.S. Pat. No. 3,556,855 discloses the use of metal-coated fibers in an electrode, but the fibers are of short length and are prepared using an electroless plating process, a method which does not provide the thickness and continuity of coating required, particularly in the case of the deposition of a lead coating, to make a grid from such a wire. The patent does disclose the mixing of a thermosetting resin with fibers coated by electroless deposition to form a grid element.
U.S. Pat. No. 4,169,911 discloses short, metal-coated carbon fibers held together by a binder for use in battery electrodes, and notes that lead is one of the metals which may be used to coat the fibers. It indicates that the fiber may be coated by electrochemical plating, chemical plating, vacuum deposition, sputtering, ion plating, plasma jet application or chemical deposition. However, none of these methods can be used to produce a lead-coated wire capable of being woven into a scrim which may be used as an electrode grid. The fibers disclosed by this reference are characterized by a conductivity which is too low, due to the thinness of the layer of metal deposited on the fiber, for such a use. Nor do the metal-coated fibers produced by such processes possess the necessary surface characteristics (i.e., uniform, small grain size) to resist corrosion in a lead-acid environment. Further, the strength of the porous material disclosed by this reference is too low to support the layer of active material of the electrode.
U.S. Pat. No. 3,776,612 discloses short carbon or asbestos fibers plated with lead for use as a bearing material. Although the conductivity of these fibers may be adequate for batteries which are used in certain applications, the relatively thin plating would be oxidized rapidly in the harsh environment of the lead acid battery, resulting in broken electrical continuity, rendering the battery useless. Further, the process disclosed by this reference for the making of such fibers is relatively slow and expensive.
The electrodeposition of metal, including lead, onto synthetic polymer fibers using a metal chloride nitrate or sulfate solution is disclosed by U.S. Pat. No. 3,940,533. Such fibers show improved conductivity and antistatic properties. Again, the thinness of the lead coating precludes the use of this material as the grid conductor of a battery. Further, the fibers disclosed would not survive the sulfuric acid and lead oxide attack of the lead acid battery.
U.S. Pat. No. 3,958,066 discloses a method of making synthetic polymer fibers with a metal powder attached to their surface for improved conductivity and antistatic applications. Such a fiber is useless in a lead acid battery because the synthetic polymer would not survive the chemical environment of the battery and because pure lead, not particles of lead, must form a continuous coating on the fiber in order to avoid rapid oxidation of the lead and to provide the necessary electrical conductivity.
U.S. Pat. No. 2,963,739 discloses an apparatus and method of applying metal to glass filaments. Glass fibers are drawn through a bead of molten metal which forms at the face of an applicator, then stranded and spooled. The apparatus is apparently intended for the coating of glass fibers with copper, aluminum, silver or alloys of these metals, but is, however, unsatisfactory for coating of lead or zinc onto metal wire or glass fibers. Both melted lead and melted zinc have high surface tensions and poor wetting characteristics. When those characteristics are added to the fact that, when melted, both oxidize easily and have a poor affinity for glass, the result is an impure and irregular coating which is unsatisfactory because of the effect of those impurities and irregularities on the electrical conductivity of the coated wire and because those impurities and irregularities expose the core of the composite wire to the acid of the lead acid battery, resulting in damage to the core. To a lesser extent, this same problem exists with regard to the coating of a core material with nickel or zinc.
Another approach which can be utilized to reduce the weight of the battery while improving its electrical characteristics is to make the conductor, or bus bar, which removes current from the grid, and/or which passes current from grid to grid within the battery, lighter and to improve its conductivity so that a proportionally smaller conducter can be utilized. Those objectives could also be accomplished by providing a composite wire with a core made of a conductive metal such as copper or aluminum and a thin coating of lead. However, so far as is known, no composite wire is available to satisfactorily provide such a reduction in weight or size.
Taking advantage of the fact that only the outermost layer of the lead of the grid element is necessary for proper function of the electrodes of a metal acid battery, the present invention provides a continuous metal coated, composite wire of small diameter. This wire may be woven into a scrim in a wide variety of shapes and sizes for use as a grid which may be used in an electrode in an electrochemical cell. Such a grid is characterized by the desired properties of strength, durability, corrosion-resistance, conductivity and light weight. It is, therefore, an object of the present invention to provide a lightweight, metal coated fiber wire of small diameter and continuous length.
It is another object of the present invention to provide a lightweight, lead coated wire with the high tensile and shear strength and corrosion resistance necessary to allow the use of the wire in the electrode of an electrochemical cell.
It is another object of the present invention to provide an apparatus and method of making a metal-coated wire, the surface of which is characterized by extremely high corrosion resistance properties arising from precisely controlled, solid-phase extrusion of the metal coating onto a core of a fibrous material, optical fiber or highly conductive metal.
It is another object of the present invention to provide a material from which a grid for an electrochemical battery may be made which will extend the life and reduce the weight of the battery.
It is another object of the present invention to provide a material from which a grid for an electrochemical battery may be made which will increase the specific energy of the battery.
It is another object of the present invention to provide a method for making a metal coated fiber.
A further object of the present invention is to provide a method for coating a fibrous material with an extrudible, corrosion-resistant metal such as lead, zinc or nickel.
It is another object of the present invention to provide a method for making a continuous length of a fine core material with a layer of lead deposited around the core material.
A further object of the present invention is to provide an apparatus for use in the coating of a fine, high tensile strength fibrous material with an extrudible, corrosion-resistant metal.
It is another object of the present invention to provide an apparatus capable of depositing a thin layer of lead, zinc, nickel or other corrosion-resistant metal around a fine core material.
It is another object of the present invention to provide an apparatus capable of depositing a thin layer of extrudible, corrosion-resistant metal around a core material, the corrosion-resistance properties of the metal being enhanced due to the uniform, small grain size of the metal, and the grains being oriented longitudinally.
It is another object of the present invention to provide an apparatus which is capable of making continuous lengths of light weight, lead coated wire of small diameter.
It is another object of the present invention to provide a light-weight, high tensile strength, metal-coated wire of small diameter, the wire having high corrosion resistance capabilities due to the uniform, small grain size of the surface of the metal, and the grains longitudinal orientation.
Still another object of this invention is to provide a light-weight, durable fabric woven from a metal coated fiber.
Another object of the present invention is to provide a fine, high tensile strength, lead-coated wire which may be woven into a scrim for use in the electrode of a lead acid battery.
Another object of the present invention is to provide a composite wire comprising a core of a variety of materials, including highly conductive metals such as copper or aluminum, and a coating of an extrudible, corrosion-resistant metal such as lead, zinc or nickel.
Another object of the present invention is to provide an improved electrode grid constructed of a scrim woven from a lightweight, high tensile strength fiber coated with lead and a bus bar constructed from a core of a highly conductive metal such as copper or aluminum and a coating of lead.
Another object of the present invention is to provide electrode grids of high conductivity, light weight and increased durability for use in an electrochemical battery.
Another object of the present invention is to provide connectors and cell interconnect cabling of high conductivity and light weight for use in an electrochemical battery.
Another object of the present invention is to provide an electrode composed of a combination of a fabric woven from a lead coated fiber and a paste of active material for use in a lead acid battery.
Another object of the present invention is to provide an electrochemical cell which may be assembled into an electrochemical battery that is light in weight, has a long battery life and has a high specific energy.
A further object of the present invention is to provide a method of making an electrode grid of high conductivity, light weight and increased durability.
Another object of the present invention is to provide a battery in which the interplate and intercellular interconnectors are eliminated.
Yet another object of the present invention is to provide a high tensile strength conductor which will pass high currents but will blow easily in response to high transient currents, thereby serving as a fusing material.
Another object of the present invention is to provide a cabled composite wire comprising several individual fine composite wires having either a fibrous material core or a highly conductive metal core and a coating of an extrudible, corrosion-resistant metal such as lead, zinc or nickel, each and all of the composite wires being contained within an extruded lead sheath.
Another object of the present invention is to provide a light-weight cloth woven from a lead coated, composite wire useful to absorb ionizing and/or electromagnetic radiation or for noise abatement.
Another object of the present invention is to provide a lead coated, composite wire having an optical fiber core for use in shielded fiber optic communications circuits.
Various other objects of the present invention and the advantages it represents will be readily apparent from the following description of the drawings of the apparatus of the present invention, in which exemplary embodiments of the invention are shown, and the description of the method of the present invention.